Although incandescent bulbs based on light emission by heated filaments have been widely used as lighting devices, they have had problems of short life, luminous efficiency, etc.
In recent years, LED light sources and EL (electroluminescence) light sources have been developed as technologies to solve these problems, and use of the LED light sources, in particular, for conventional lighting devices have been exponentially spread.
In general, the external shape of an LED bulb is defined by a cap attached to a metallic base member and a hemispherical light-transmitting cover, and a mounting substrate mounted with an LED light source in a position corresponding to the spherical center of the light-transmitting cover is attached to the base member. The light source is caused to emit light through a drive circuit in the base member by electricity supplied through the cap.
Light from the light source mounted on the mounting substrate has such directivity that the luminous intensity is attenuated in proportion to cos θ, where θ is an angle between the normal direction of the mounting substrate and strong light emitted normal to the mounting substrate. This is because the conventional LED light source is configured so that an LED chip that emits a primary light beam is covered flat by a protective layer containing a phosphor that converts the primary light beam into a secondary light beam. Thus, a lighting device using an LED light source for a bulb or fluorescent lamp has such a luminous-intensity distribution that light normal to the mounting substrate is strong and hardly any light is emitted laterally or to the rear relative to the mounting substrate. If a conventional incandescent bulb or fluorescent lamp that has a substantially uniform luminous-intensity distribution from front to back is replaced with the lighting device using the LED light source, therefore, the brightness of the ceiling and walls is inevitably greatly changed, resulting in a differently illuminated space.
A technique in which LEDs that constitute a light source are laterally and rearwardly disposed in a three-dimensional manner is proposed as a technique to solve the problem of the narrow luminous-intensity distribution. As another technique, moreover, there is a technique in which the inner surface of a light-transmitting cover is coated with a phosphor that can be excited by light from an LED light source, whereby the light-transmitting cover itself glows. Still another technique is proposed in which a light source is disposed at the lower end of a spherical light-transmitting cover. There is still another technique in which a light guide body is installed near an LED light source.
If an LED light source is mounted three-dimensionally, there are problems that the manufacture and assembly of a lighting device are complicated and the difficulty of the design for mechanical strength and thermal dissipation inevitably increases. Also if a phosphor is applied to a light-transmitting cover, there is a problem that the manufacture and assembly of the lighting device are complicated. If the light source is disposed at the lower end of a spherical light-transmitting cover, a base member is made shorter or smaller than the overall length restriction of the lighting device, so that thermal radiation is inevitably degraded and fails to produce a large amount of heat. If a light guide body is installed, moreover, the prior art techniques can provide neither a sufficient luminous-intensity distribution control function nor a natural design.